In voice/audio digital processing, data streams may come from different sources that are not synchronized with the receiver clock. The frequency difference due to the differences between the remote and the local systems crystal oscillator frequencies is usually very small, such as 100 parts per million (ppm). One method of handling a frequency discrepancy is to use a voltage controlled crystal oscillator (VCXO) to lock the local system frequency to the remote system frequency. However, this technique is limited to a single received frequency and can not be used for multiple unrelated incoming data streams.
Further, in portable applications low power dissipation is also a major design constraint. Thus, it is ideal for a system with multiple arriving signals to have a single voltage controlled oscillator (VCO) operating at the least possible frequency.